JS how to flatten a nested array

Question

The question I have been given is;

Given a nested array or arrays, return a new, flat array with all the elements of all the nested arrays in their original order

The answer that I have come up with is;

function flattenArray (arrayOfArrays) {  
arrays = arrays.reduce(function(a, b){
    return a.concat(b);
}, []);

console.log(merged);

My answer is being tested against this;

describe("flattenArray", () => {
it("returns a flat array with all the elements of the nested arrays in      their original order", () => {
  let arrayOfArrays, expected;
  arrayOfArrays = [[1, 2], [], [3], ["hello", true]];
  expected = [1, 2, 3, "hello", true];
  expect(flattenArray(arrayOfArrays)).to.eql(expected);

  arrayOfArrays = [[1], [2], [[3, 4]]];
  expected = [1, 2, [3, 4]];
  expect(flattenArray(arrayOfArrays)).to.eql(expected);
});
it("does not mutate the passed array, i.e. returns a new array, leaving the original untouched", () => {
  const original = [[1, 2], [], [3, 4]];
  const flat = flattenArray(original);
  expect(original).to.not.equal(flat);
  expect(original).to.eql([[1, 2], [], [3, 4]]);
});
});

I have no clue how else to try and solve this question, does anyone have any sugestions.

do you need to flat only one level, or more? btw, you need to return the reduced value in the function. — Nina Scholz
– Nina Scholz, Commented Dec 19, 2018 at 17:26
Although not supported by IE, on good browsers you can use Array.prototype.flat() — Eponyme Web
– Eponyme Web, Commented Dec 19, 2018 at 17:31

Nina Scholz · Accepted Answer · 2018-12-19 17:34:27Z

1

You need to return the reduced array by taking the handed over array arrayOfArrays.

function flattenArray(arrayOfArrays) {  
    return arrayOfArrays.reduce(function(a, b) {
        return a.concat(b);
    }, []);
}

For multiple nested arrays, you need to check for array and use a recursion of the function.

function deepFlattenArray(arrayOfArrays) {
    return arrayOfArrays.reduce(function(a, b) {
        return a.concat(Array.isArray(b) ? deepFlattenArray(b) : b);
    }, []);
}

function flattenArray(arrayOfArrays) {
    return arrayOfArrays.reduce(function(a, b) {
        return a.concat(b);
    }, []);
}

console.log(deepFlattenArray([[[1, 2], [3, [4, 5], 6], 7], 8]));
console.log(flattenArray([[[1, 2], [3, [4, 5], 6], 7], 8]));

.as-console-wrapper { max-height: 100% !important; top: 0; }

edited Dec 19, 2018 at 17:34

answered Dec 19, 2018 at 17:28

Nina Scholz

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Comments

Scott E · Accepted Answer · 2018-12-19 18:04:46Z

1

Perhaps not the most elegant solution, but this will flatten any number of nested arrays. It's a recursive function that modifies a new array as a side-effect:

var arrOfArrs = [[1, 2, 3], [4, 5, 6], 7, [8, [9, 10, 11, 12]]];
newArr = [];
function flattenArray(arr) {
    for(var i=0; i < arr.length; i++) {
        typeof arr[i] == 'object' ? flattenArray(arr[i]) : newArr.push(arr[i]);   
    }
}

flattenArray(arrOfArrs);
console.log(newArr);

answered Dec 19, 2018 at 18:04

Scott E

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Comments

Jonathan Ramos · Accepted Answer · 2018-12-20 16:50:22Z

1

You can use reduce plus spread operator. Just coded this for the particular question but seems to work ok. It uses recursion and works for Multiple nested arrays.

function flatArray(a){
  return a.reduce( (accumulator, current) => {
    if (!Array.isArray(current)) return [...accumulator, current];
    return flatArray([...accumulator, ...current]);
  }, []);
}

let a = [ 1, [2], [3,4], [], [5], [6, [7]], [[[8]]] ];

console.log(flatArray(a));

edited Dec 20, 2018 at 16:50

answered Dec 19, 2018 at 18:34

Jonathan Ramos

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